The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
It is already known, in particular in the field of aeronautics, to use composite panels having a sandwich structure formed by a central core having a cellular structure, for example of the honeycomb type, sandwiched between two skins.
These composite panels may also be acoustic attenuation panels designed to reduce noise emissions from turbojet engines, these panels generally having a sandwich structure comprising:
an outer (oriented towards the source of the noise) air-permeable perforated skin, called “resistive” or “acoustic” skin, whose role is to dissipate the acoustic energy;
a central core having a cellular structure of the honeycomb type; and
an inner skin formed by a solid skin (opposite to the source of the noise), called structuring skin.
In some cases, the composite panels must be designed to be installed in the hot area of an aircraft turbojet engine nacelle, and in particular in the downstream portion of this nacelle through which exhaust gases are expelled.
The use of such composite panels is generally structural and, in the case of acoustic attenuation panels in this exhaust area, this also allows substantially reducing the sound emissions located in the high-frequency range.
Moreover, in order to resist some structural constraints, it is known to use several composite panels, some of which, located at the most structurally stressed constraint areas, comprise a central core having a denser cellular structure than that of the adjacent panels.
Another alternative for structurally reinforcing a composite panel is to join, edge-to-edge, two cellular structures of a different density, the area intended to be the most stressed being provided with a cellular structure of a greater density, for example by means of cells of smaller size or by means of thicker thickness of the walls delimiting the cells.
However, these solutions have in particular the disadvantage of requiring one or more junction(s) between two distinct cellular structures, or even between two composite panels. These junctions constitute areas of weakness in case of strong mechanical and thermal stresses, but also upon the installation of through-fasteners.
Moreover, for the particular applications in the field of aeronautics, and in particular in the use of such composite panels on nacelles of aircraft turbojet engines, said panels generally present reliefs such as curvatures, that is to say they are not planar.
In this context, the forces exerted on junctions between two distinct cellular structures, or even between two distinct composite panels, at such curvatures tend to further weaken the structural assembly.